1. Field of the Invention
This invention concerns high temperature resistant resin coatings and more specifically concerns water-thinned high temperature resistant polyimide resin coating compositions together with methods of their preparation and use.
2. Brief Description of the Prior Art
Polyimide resins, polyamide-imide resins and their precursor resins have been employed heretofore in the preparation of high temperature resistant varnishes and enamels; see for example, U.S. Pat. Nos. 2,880,230; 3,179,614; 3,179,633; 3,179,634; 3,451,848; 3,518,219; 3,541,036; 3,546,152; 3,582,458 and 3,702,788.
Although some of the polyamide acid resins employed as precursors for preparing polyimide resins have been previously disclosed as forming water soluble salts (see for example U.S. Pat. No. 3,507,765) such polyamide acids are substantially free of imide linkages and are of relatively low molecular weights. The higher molecular weight polyimide and polyamide-imide polymers containing significant proportions of polyimide moieties in their backbones have heretofore been recognized as relatively insoluble in water. For this reason, polyamide-imide and polyimide resins have not been employed in water-thinned paint compositions although it is desirable to do so in view of the better physical properties generally obtained in coatings prepared from the relatively higher molecular weight polyimide resins.
I have found that certain polyamide-imide resin precursors of polyimides, having free carboxyl groups, will form salts with amine bases and that unexpectedly such salts are readily soluble in aqueous media. Moreover, I have found that these water soluble resin salts may be employed to prepare water-thinned coating compositions of the invention which have a number of advantageous properties. For example, the compositions of the invention are easily applied, exhibit excellent flow properties, resist degradation upon exposure to high temperatures and obviate the need for toxic and ecologically undesirable organic volatiles as thinning agents. Further, the ready water solubility of the resin salts employed enable one to employ relatively high molecular weight polymer resins, with their associated improved curing times and coating characteristics. The compositions of the invention may be used in high resin concentrations, i.e.; up to at least 30 percent resin contents.